Electronic display device include a light-emission type and a light-receiving type. Examples of the light-emission type include a CRT (cathode ray tube), a PDP (plasma display), an ELD (electroluminescent display), a VFD (fluorescent display tube) and a LED (light-emitting diode).
Among them, the LED will be described below.
The LED is a light emission element comprising a light emission material emitting light in an electric field or combination of several number of such the element. The elements are classified into an organic element and an inorganic element according to the material and into a carrier injection type and an accelerated electron type according to the light emission mechanism. The recombination of an electron and a positive hole is utilized in the carrier injection type element and collision energy of an accelerated electron is utilized in the accelerated electron type element. Generally, the inorganic material is longer in the life time and more stable than the organic material. However, it is a shortcoming of the inorganic material that the choice of the material is narrow and there is a limitation on the molecular design thereof. The recombination type has advantage that the driving voltage is lower than that for the electron accelerate type. Recently, therefore, the carrier injection type LED is extensively developed.
The LED include the following three types.
(1) Inorganic LED comprising a inorganic compound such as GaN and GaInN: the light emission mechanism thereof is recombination type. It is simply called also as LED (light emission diode).
(2) Organic LED comprising an organic compound such as a triarylamine derivative and a stilbene derivative: the light emission mechanism thereof is recombination type. It is called as an organic EL (electroluminescent or OLED.
(3) Inorganic EL comprising an inorganic material such as ZnS:Mn and ZnS:Tb: the light emission mechanism is the accelerated electron type. It is called simply as an electroluminescent element since the element of such the type is historically old.
The “electroluminescent material” in the invention includes the above-mentioned (1) and (2). Therefore, (3) is not subject of the invention.
In the field of the carrier-injection type organic electroluminescent element which has been particularly noted in recent years, ones emitting high luminance light have been becoming to be obtained after a thin layer of organic compound has been used. For example, U.S. Pat. No. 3,530,325 discloses one using a single crystal of anthracene as the light-emission substance, Japanese Patent Publication Open for Public Inspection (JP O.P.I.) No. 59-194393 discloses one having a combination of a positive hole injection layer and an organic light emission layer, JP O.P.I. No. 63-295695 discloses one having a combination of a positive hole injection layer and an organic electron injection layer, and Jpn. Journal of Applied Physics, Vol. 127, No. 2, p.p. 269-271, discloses one having a combination of a positive hole transportation layer and an electron transportation layer. The luminance of emission light is improved by such the means.
Besides, a fluorescent substance has been known, which emits fluorescent light by absorbing light emitted from the electroluminescent material. The method using such the fluorescent substance to emit various colors light by means of an electroluminescent material is applied for the CRT, PDP, VFD, etc. However, in such the case, there is a problem that light emitted from the electroluminescent material must be a high energy ray (i.e., short wavelength emission) such as an electron ray or a far ultraviolet ray. The fluorescent substances described above are essentially inorganic fluorescent substances. There are known a number of the inorganic fluorescent substances which are superior in stability, exhibiting long shelf-life. However, there has not been found a long wavelength excitation type inorganic fluorescent substance exhibiting an excitation wavelength in the region of near ultraviolet to visible light, specifically, red light.
A near ultraviolet ray capable of being emitted from the electroluminescent material is contemplated to be a light having a peak of wavelength within the range of from approximately 350 nm to 400 nm, and the use of an organic fluorescent dye as the fluorescent substance capable of excited such the near ultraviolet ray is disclosed in JP O.P.I. Nos. 3-152897, 9-245511 and 5-258860.
However, it is known that the organic fluorescent dye is generally tends to be influenced by the circumstance condition, for example, change in the wavelength or quenching tends to be occurred depending on the kind of solvent or medium such as a resin.
In the methods disclosed in the foregoing publications, a fluorescent dye which absorbs light of blue or blue-green light region emitted from the electroluminescent material and converts the light to red light. A fluorescent conversion layer which emits light in green region has characteristics that the Stokes shift (the difference between the wavelength of the absorbed light and that of the emitted light) is small, and a part of light emitted from the electroluminescent material can be permeated therethrough, and the light from the light emission material can be converted with a relative high efficiency. However, the conversion to the fluorescent to light of red region caused problems that the conversion efficiency is considerably low since a large Stokes shift is needed and the light from the light emitting material almost cannot be utilized. Exemplarily, the combined use of a few fluorescent dyes different in excitation wavelength is needed and it is necessary to utilize light-to-light conversion (i.e., photoluminescence) of plural fluorescent dyes, such as a fluorescent dye emitting yellow light in response to blue light and a fluorescent dye emitting red light in response to yellow light.
Accordingly, there is a problem that the visual perceivability and the luminance of color displaying by such the element are lowered since the luminance balance between blue, green and red light emission is unsuitable and the above-mentioned quenching and decoloration are occurred.
The inventors can obtain an electroluminescent element capable of emitting a high luminance light and having a high storage ability, and can provide a color filter with a high luminance by the use of such the electroluminescent element.